writeback: fix typo in comment

Spelling mistake (triple letters) in comment.
Detected with the help of Coccinelle.

Link: https://lore.kernel.org/r/20220521111145.81697-32-Julia.Lawall@inria.fr
Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>

Signed-off-by: Jan Kara <jack@suse.cz>